As disclosed in U S. patent application Ser. No. 318,133 filed Mar. 2, 1989 in the name of Mark Schutts, assigned to the present assignee, and now U.S. Pat. No. 5,021,737, proximity sensors are used in conjunction with rotating machinery to sense imbalances like position change of, for example, a shaft. Such sensor includes adjacent to the shaft a sensing coil and at the other end a cable connected to an electrical processing unit. Such processing unit, which normally will include an oscillator, senses the movement or displacement of the rotating machinery by a change in amplitude of the oscillating signal. Any change in amplitude is primarily because of a change in the inductance and resistance of the coil which is affected by the displacement or vibration of the rotating machinery.
Because of space restrictions for the proximity sensor and its coil, the coil should have a diameter less than 0.3 inches. At the same time since the coil is subject to a temperature range of -34.degree. C. to +177.degree. C. it should have a relatively low temperature coefficient of electrical resistance. Also, since a relatively high operating frequency, for example, 1 megahertz, is used, the specific electrical resistance should be low and yet have sufficient inductance. Too high a resistance or too low an inductance will prevent the oscillator from functioning.
One wire material which was used for a probe coil was 100% silver wire which had a temperature coefficient of 3800 PPM/.degree. C. at a resistivity of 9.8 ohms per cir.mil foot. For this type of Wire, it Was known that the AC resistance changed contrarily in an opposite mode to the DC resistance so that by selecting an optimum operating frequency, the error caused by temperature changes could be somewhat canceled out. However this still might have amounted to 45% error.
Another material which has been used commercially is a copper alloy wire with 98% copper and 2% nickel. This material had a temperature coefficient of 1400 PPM/.degree. C. and resistivity of 30 ohms per cir. mil foot. This had an error of about 30%. This material while suitable for some purposes had at higher operating temperatures undesirable characteristics.
If coil diameter is not a problem, then materials with higher resistivity and small temperature coefficients, can be used. One type is a copper nickel alloy with 55% copper and 45% nickel having a temperature coefficient of 20 PPM/.degree. C. and a resistivity of 300 ohms per cir. mil foot. Another type, as shown in U.S. Pat. No. 4,517,156, is a palladium /silver alloy With about 60% palladium and 40% silver. This wire has a coefficient of 20 PMM/.degree. C. and a resistivity of 240 ohms per cir. mil foot. Such higher resistivity wires are not suitable for small diameter coils. Small diameter, high resistivity wires would result in too high a resistance to allow the oscillator to function properly. Large diameter, high resistivity wires wound on a small diameter coil would not allow sufficient inductance to allow the oscillator to function properly.